Molybdenum compounds of high purity are required for various applications such as in catalysts, etc.
Heretofore there have been a number of procecesses for upgrading relatively impure molybdenum compounds.
U.S. Pat. No. 3,957,946 discloses a process for purifying molybdenum which involves subjecting impure concentrates of molybdenum trioxide to oxidative roasting, followed by ammoniacal leaching of the molybdic trioxide to produce ammonium molybdate which is further purified by passage through a chelating cation exchange resin.
U.S. Pat. Nos. 3,848,049 and 3,829,550 disclose purification processes for molybdenum which involve leaching of molybdenum trioxide with dilute nitric acid.
U.S. Pat. No. 3,694,147 discloses a process for purifying molybdenum trioxide of impurities such as lead, copper, iron, and zinc and some alkaline earths by leaching with an aqueous solution and an ammonium salt of the halide.
U.S. Pat. No. 3,860,419 relates to the process for recovering molybdenum from molybdenum concentrates by digesting the concentrate in an aqueous nitric acid solution containing ammonium nitrate, separating the solid phase from the liquid phase, and treating the solid phase with ammonium hydroxide to produce ammonium molybdate.
U.S. Pat. No. 4,079,116 relates to a process for producing high purity ammonium molybdate, ammonium heptamolybdate or ammonium dimolybdate from a molybdenum oxide concentrate by a series of operations which involve cation removal, leaching, etc.
In the above processes, some impurities remain to contaminate the product.
Potassium in molybdenum can interfere with sintering, resulting in low density. Potassium is a particularly difficult impurity to remove. When U.S. Pat. No. 3,860,419 is followed for purifying molybdenum, the potassium is not sufficiently reduced for use as high purity material.
U.S. Pat. No. 3,393,971 relates to a process of re-roasting molybdenum trioxide followed by water washing the molybdenum trioxide and then forming an ammonium molybdate (AM) solution from which ammonium paramolybdate (APM) is produced. The APM is then oxidized to produce pure molybdenum trioxide.